1. Field of the Invention
This invention relates to an improved arc grid for use with an electrical switching device and, more particularly, to such a grid having improved arc grid plates for use with a circuit interrupter such as an electromagnetic contactor. The invention also relates to an electrical switching device having improved arc grid plates.
2. Background of Information
Electrical switching devices include, for example, electromagnetic switching devices such as circuit breakers, circuit interrupters and other circuit switching devices. Circuit interrupters, for example, are electrically operated switches used for controlling motors and other types of electrical loads. Circuit interrupters include, for example, contactors, motor starters, motor controllers and other electromechanical switching devices. Electromagnetic contactors, for example, include a set of movable electrical contacts which are brought into contact with a set of fixed electrical contacts to close the contactor. The set of movable contacts is separated from the set of fixed contacts to open the contactor. One of the fixed contacts is connected to a line terminal and the other of the fixed contacts is connected to a load terminal. The line terminal is connected to a power source and the load terminal is connected to a load in order to selectively source current from the power source to the load.
During a contact breaking cycle, pairs of fixed and movable contacts part. The current flowing through each pair of contacts forms an arc therebetween. The arc erodes the contacting surfaces of the contacts until the arc is extinguished. In order to minimize the arc between the contacting surfaces, a plurality of parallel arc plates are disposed between a first conductor which is electrically connected to the fixed contact and a second conductor which is electrically connected to the movable contact. In this manner, the arc is primarily directed between the first conductor, the arc plates, and the second conductor without adversely eroding the contacting surfaces.
The pair of contacts and the first and second conductors are housed within an arc chamber of an arc box. It is known to position a plurality of parallel, generally rectangular arc plates within the arc chamber between the conductors. The sides of the arc box have a plurality of opposing grooves in which the sides of the arc plates rest. It is also known to provide a plurality of foot-shaped grippers on each of the sides of the arc plates in order to retain each arc plate in a pair of the opposing grooves. Each of the grippers has a heel disposed along the side of each arc plate and a toe pointing outwardly from the side. Each arc plate is inserted heel first into the pair of opposing grooves in order that the toes flex inwardly away from the edges of the grooves thereby resisting, but not preventing, insertion of the arc plate. After each arc plate is fully inserted within the pair of opposing grooves, the toes dig into the edges of the grooves when a force is applied to remove the arc plate.
During operation of the contactor, the arc erodes a front edge of the arc plates which is closest to the current path of the arc between the first conductor and the second conductor. During rapid operation of the contactor at moderate to high currents, the arc also erodes the molded material of the sides of the arc box closest to the arc. As the sides of the arc box erode, the toes loosen their grip on the edges of the opposing grooves. In turn, the arc plates and the arc tend to slip toward the pair of contacts causing the temperature of the contacting surfaces to rise, thereby accelerating the erosion of the contacts and reducing the useful electrical life thereof.
There is a need, therefore, for an improved arc plate for a contactor which decreases the erosion rate of the separable contacts.
There is a more particular need for such an arc plate having grippers which maintain the grip on the edges of the opposing grooves of the arc box.
There is an even more particular need for such an arc plate which is easier to install within the opposing grooves of the arc box.